The present invention relates generally to a backlight for a display and, in particular, to a light emitting diode (“LED”) backlight for a display that is compatible with night vision systems.
Graphic displays, such as those employing a liquid crystal display (“LCD”) screen provide a field of pixel elements, each of which may be independently controlled to block or pass light, for example, from an underlying backlight.
A common backlight for use with an LCD screen provides a transparent panel edge lit by one or more fluorescent tubes. A reflective rear surface of the panel directs the edge illumination towards an LCD screen positioned against a front surface of the panel. The reflective rear surface of the panel may be gradated to produce an even field of illumination behind the LCD screen to compensate for an inherent falloff of brightness attributable to a distance of the fluorescent tube. In this regard, fluorescent tubes provide a relatively high efficiency light source providing a broad color spectrum output suitable for backlighting LCD screens.
However, in some applications, fluorescent tube backlights have a number of disadvantages. For example, fluorescent tube backlights generally require a high voltage power supply, include fragile glass tubes, have a tendency to fail unexpectedly, and have a limited ability to change brightness levels. In avionics and other demanding applications, where these drawbacks are substantial, LED backlights may be preferred.
In avionics, night vision systems (“NVS”) or night vision imaging systems (“NVIS”) are sometimes employed. These NVSs work by collecting photons and passing the photons through an image intensifier assembly to create an image that is visible to a user in a micro-display worn by the user. A given NVS may amplify the nighttime scene approximately 2000 times. Accordingly, NVSs must be used in an environment that is substantially free of stray cockpit light in the NVIS band. In particular, NVSs have a very high sensitivity to radiation in the region of approximately 620 nanometers (nm) to 930 nm (orange to infrared), thus stray infrared (“IR”) illumination must be controlled or else a “bloom” will be experienced where the image intensifier assembly is overloaded and the image shown on the micro-display becomes unviewable.
In some cases, NVSs may be equipped with an automatic gain control (“AGC”) that will decrease the sensitivity of the NVS when exposed to high radiation in the region of approximately 620 nm to 930 nm to protect the image intensifier assembly and the user from experiencing a bloom. In this regard, if, for example, displays or light sources in an airplane cockpit emit high radiation in these regions, the AGC may activate and the NVS will become proportionally less sensitive to nighttime objects outside of the cockpit.
In order to protect against blooms and/or decreased sensitivity caused by AGC functions, night vision compatibility standards, such as NVIS standards, have been developed to guide the design of lighting equipment that can be used with and without NVSs. Under avionics NVIS standards, overlap between the emissions spectrum of the display or backlight and the spectral response of the NVS is minimized. There are also NVIS standards for ground vehicle operations where lower or near infrared transmittance need not be eliminated, but should be significantly reduced to, for example, 5 percent of the total visible component.
To meet such standards, filters are commonly used, which reflect or absorb radiation that could interfere with the operation of NVS while allowing visible light to pass. For example, a “hot mirror”, positioned between the backlight and LCD screen, which rejects the undesired infrared rays, but lets visible light pass through, is also often used to filter displays for use with NVSs. By significantly reducing IR radiation in electronic display and lighting systems, NVSs can be used along with such electronic display and lighting systems without affecting the nighttime sensitivity of the NVSs.
Unfortunately, these filters add significantly to the cost of the LCD screens. Furthermore, these filters are generally permanently affixed to the LCD screen decreasing the intensity of a display by approximately 10 to 20 percent, which corresponds to a significantly dimmed screen when viewed in conditions such as daylight. Furthermore, these filters often impede portions of the visible spectrum range and/or viewing angle in which case, the LCD screen may appear “unnatural” to an operator especially when viewed in daylight. The effectiveness of an interference type filter varies at different viewing angles, causing further loss of visible light.
To overcome these shortcomings, the power delivered to drive the display is often increased. While this may compensate for the decreased intensity, the additional heat generated by the increased power consumption places strain on cooling systems. Furthermore, the additional power consumption may be undesirable in many applications such as avionics when a fixed supply of power is available.
It would therefore be desirable to have a cost-efficient LED backlight for a display that is compliant with night vision viewing standards and is capable of performing sufficiently during periods when night vision systems are not employed.